Devices using various types of media such as optical disks, magnetic tapes, and semiconductor memories are known in the art as data storage devices. Among them, hard disk drives (hereinafter referred to as HDD) have become popular as storage devices for computers to such an extent that they are one of the storage devices indispensable for today's computer systems. Further, not limited to the computers as described above, HDDs are expanding more and more in application. For example, HDDs are used for moving picture recording/reproducing devices, car navigation systems, cellular phones, and removable memories for use in digital cameras.
The magnetic disks used in the HDD have a plurality of concentrically formed data tracks. A plurality of sets of servo data, each set including address information, and a plurality of data sectors each including a user data region are recorded on each data track. More than one data sector is recorded between the individual sets of servo data. The head element section of a head slider supported by an actuator which swings can access a desired data sector in accordance with the address information of the servo data, thus writing user data onto or reading out user data from the data sector.
To improve the recording density of a magnetic disk, it is important to reduce the clearance between the magnetic disk and the head element section flying above the surface of the magnetic disk. In order to achieve the reduction, several mechanisms for adjusting the clearance are proposed. One of the mechanisms is by providing the head slider with a heater and heating the head slider with this heater to adjust the clearance (refer to Japanese Patent Laid-Open No. 05-20635, for example). Hereinafter, this method is referred to as TFC (Thermal Flyheight Control). In TFC, an electric current is supplied to the heater to generate heat and make the head element section 12 protrude by its own thermal expansion. The clearance between the magnetic disk and the head element section is thus reduced.
If an error occurs during internal processing of the HDD, the HDD executes an error recovery process (ERP). The HDD has several kinds of ERPs, such as an ERP for the reading process of reading out user data from the magnetic disk, an ERP for the writing process of writing user data onto the magnetic disk, and an ERP for the process of reading out microcodes from the magnetic disk. Each ERP includes multiple ERP steps for modifying circuit parameters and target positions. The HDD achieves error recovery by executing each ERP step, and the ERP ends in the step where the HDD has recovered from the error.
Important in the ERP is to achieve error recovery in the earliest possible step and shorten the processing time required. A delay in error recovery reduces performance. In addition, if the processing time is too long, a host will judge the HDD to be inaccessible, and as a result, the connection to the HDD will be disconnected as a time-out error (time out). It is therefore required that the appropriate ERP step be executed and that each ERP step be executed in the appropriate order.
Regarding the HDD, it is important to suppress the instability of the head characteristics. The instability appears as changes in the reading signal waveform of the reading element. More specifically, the deterioration of the instability increases reading signal noise, causing positive or negative amplitude of the reading signal to increase significantly and become asymmetrical. These events hinder accurate readout of servo data or user data. This is due to the fact that the free layer in the reading element does not have a single-domain structure and partly includes a small magnetic domain which exhibits a magnetic behavior different from that of other regions forming the layer.
The deterioration of the instability is therefore caused by such as factors such as the insufficient magnetic bias of a bias layer or the asymmetry of the magnetic bias applied from the hard bias layers formed at both sides of the free layer. Even when the instability does not manifest itself during the manufacturing phase of the HDD, if the bias layer is not completely formed during the manufacturing phase, electrostatic discharge (ESD), head-disk contact, or other events will deteriorate the instability after product shipping. It is therefore important to suppress the instability when the HDD is used.